The Colorado River Basin Regional Water Board conducted bioassessment sampling at high-quality (i.e., “reference”) streams in their region during 2013-2015. The purpose of the study was to characterize the biological condition of these streams, which had never been done before, and to expand ongoing research as to whether existing biological indices, developed primarily for perennial streams, apply in nonperennial streams. The latter are usually excluded from bioassessment surveys, but make up the majority of stream length in the most arid parts of the state like the Colorado River Basin, leaving stream condition assessments in such regions largely incomplete. The indices evaluated were the California Stream Condition Index (CSCI) based on benthic macroinvertebrates and the first to have statewide applicability in perennial streams, and the D18 and S2 indices for diatoms and soft algae, respectively, developed for use in perennial streams of southern coastal California. Sixteen sites were sampled during the 3-year period, and data loggers that measure water depth were deployed for 2 of the 3 years to characterize the hydrologic regime at each site. Most sites were nonperennial, but a few were spring-fed perennial oases. Fourteen of the sites passed formal reference screening criteria developed by statewide programs.
The CSCI and S2 indices had generally good applicability in the region, with most samples from reference sites indicated either good or fair (i.e., non-degraded) biological condition. The mean CSCI score at regional reference sites was slightly lower than the statewide mean (i.e., 0.90 vs. 1.0, respectively), but the difference could be an artifact of a relatively small regional data set. The fact that CSCI and S2 performed as well as they did in a region, and in a set of stream types, largely excluded from their development indicates that the indices can be used in nonperennial streams (and spring-fed oases) in the Colorado River Basin as long as certain minimum flow conditions are met, such as sustained flow for several weeks prior to sampling and sufficient wetted width to employ existing protocols developed for perennial streams. By contrast, the D18 index did not perform well in the region. A majority of samples from reference sites indicated poor or very poor biological condition when condition thresholds based on statewide data sets were used, but even when more relaxed thresholds based on just south coast reference sites were applied, the number of reference sites in good or fair condition was still much lower than expected. This was not a result of regional reference sites being of low quality, but was most likely an artifact of applying the index to a set a stream types unintended for its use. Development of statewide algal indices is underway, which may improve the performance of diatom indices in the southern desert.